The present invention relates to a method for continuously producing a propylene-based block copolymer, which enables the stable continuous production of a propylene-based block copolymer having an excellent impact strength for a long time.
As a method for improving an impact strength of crystalline polypropylene, have been known (1) a method of copolymerizing a small amount of an xcex1-olefin; (2) a method of mechanically blending a rubber component; and (3) a method wherein a polymerization using propylene as a main component is effected, then propylene and an xcex1-olefin other than propylene are copolymerized, namely, so-called a block copolymerization is effected.
However, it seems that in the method (1), although an improvement effect in an impact strength is fairly good, a stiffness is considerably decreased and, therefore, a balance between the stiffness and the impact strength is disadvantageously lowered. While, in the method (2), although a balance between stiffness and impact strength is improved, a rubber to be used in a blend is generally expensive, so that the method is disadvantageous in the aspect of a cost.
On the basis of the background as described above, as a method for improving an impact strength of crystalline polypropylene, (3) the method of block copolymerization is most frequently used. The block copolymerization comprises the generation of a rubbery component comprising a block of a copolymer of propylene and ethylene in the presence of a resinous component comprising a block mainly comprising propylene. Compounding of both components is effected in the subsequent polymerization and, therefore, such a subsequent polymerization is also called chemical blending and used valuably. In effecting this block copolymerization, inventors found that a propylene-based block copolymer having a decreased gel generation due to a poor dispersion of a rubber component and also having a desirable impact strength could be produced at a low cost by using a magnesium compound-deposition type highly active catalyst having a sharp particle size distribution and also using a classification system comprising a precipitating liquid classifier and a condenser, and further making an active hydrogen compound co-existent in a second polymerization step. The inventors already proposed these findings (JP-A-10-120741) (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent application).
In accordance with the present invention, there is provided a method for continuously producing a propylene-based block copolymer, which enables the stable continuous production of a propylene-based block copolymer for a long time without causing accumulation of particles having a small diameter.
The first gist of the present invention resides in a process for continuously producing a propylene-based block copolymer characterized by comprising a first polymerization step wherein an xcex1-olefin comprising liquid propylene as a main component is polymerized in the presence of hydrogen and a stereoregular polymerization catalyst in one or more polymerization reactor, and a second polymerization step wherein the polymer obtained by the first polymerization step is fed to a copolymerization reactor where propylene and an xcex1-olefin other than propylene are copolymerized substantially in a gaseous phase under the stereoregular polymerization catalyst used in the first polymerization step; and satisfying the following conditions:
(1) Most part of the slurry, discharged from the polymerization reactor in the first polymerization step, containing a large amount of particles having a small particle diameter is recycled to the initial polymerization reactor by means of a classification system;
(2) A part of the polymer slurry, discharged from the polymerization reactor in the first polymerization step and classified, containing a large amount of particles having a small particle diameter is transferred to a low pressure gas post treatment system;
(3) The polymer slurry discharged from the final polymerization reactor in the first polymerization step is transferred to the second polymerization step; and
(4) The copolymerization of propylene and the xcex1-olefin in the second polymerization step is effected in the presence of a deactivating compound.
It was found by the present inventors that a part of a slurry containing a large amount of particles having a small particle diameter in a classifier is fed to a low pressure gas post treatment system and also fed to a second polymerization step, and generating dust is made to flow out at an appropriate time and in an appropriate amount so that the accumulation of the particles having a small particle size in a reactor can be suppressed.
Particularly, as compared with a case where a slurry containing a large amount of particles having a small particle diameter is fed to the post treatment system of a low pressure gas, in a case where a slurry containing a large amount of particles having a small particle diameter is fed to a second polymerization step, there occurs a fear that a short-passed highly active catalyst generates particles containing a high rubber content, i.e., so-called an increased gel content, to result in the decrease in an impact strength. However, it was further found that by estimating an appropriate bypass amount of the catalyst, a stable production of a propylene-based block copolymer could be realized and that an unreacted high-pressure PPY could be effectively recovered. On the basis of these findings, the present invention could be completed.
The gist of the third aspect of the present invention in the above-described first embodiment of the present invention resides in the process for continuously producing a propylene-based block copolymer, wherein a part of the slurry containing a large amount of particles having a small diameter is fed to the post treatment system of a low pressure gas and the second polymerization step.
Further, the present invention could be achieved on the basis of the founding as set forth below: Namely, the reactor in the first polymerization step is converted to two reactors which are connected in series, whereby even in a propylene-based block polymer having a further higher rubber content and a further higher molecular weight, small amount of gel formation and a desirable impact strength can be maintained. Further, a polymer amount load to one polymer reactor can be reduced so that respective reactors can be made small. Therefore, in a production plant which is desired to be enlarged, the size of respective reactors can have a size within a production limit.
Namely, the gist of fourth aspect of the present invention resides in a process for continuously producing a propylene-based block copolymer characterized by comprising a first polymerization step wherein an xcex1-olefin comprising liquid propylene as a main component is polymerized in the presence of hydrogen and a stereoregular polymerization catalyst in two polymerization reactors, and a second polymerization step wherein the polymer obtained by the first polymerization step is fed to a copolymerization reactor where propylene and an xcex1-olefin other than propylene are copolymerized substantially in a gaseous phase under the stereoregular polymerization catalyst used in the first polymerization step; and satisfying the following conditions:
(1) The polymer slurry discharged from the first polymerization reactor in the first polymerization step is classified by means of a classification system into a slurry containing a large amount of particles having a large particle diameter and a slurry containing a large amount of particles having a small particle diameter, and the slurry containing a large amount of particles having a large particle diameter is fed to the second polymerization reactor, while most part of the slurry containing a large amount of particles having a small particle diameter is recycled to the first polymerization reactor;
(2) A part of the polymer slurry, discharged from the first polymerization reactor in the first polymerization step and classified, containing a large amount of particles having a small particle diameter is fed to the post-treatment system of a low pressure gas;
(3) The polymer slurry discharged from the second Polymerization reactor in the first polymerization step is transferred to the second polymerization step; and
(4) The copolymerization of propylene and the xcex1-olefin in the second polymerization step is effected in the presence of a deactivating compound.
The gist of the sixth aspect of the present invention in the above-described fourth embodiment of the present invention resides in the process for continuously producing a propylene-based block copolymer, wherein a part of the slurry discharged from the first polymerization reactor in the first polymerization step and classified, containing a large amount of particles having a small diameter is transferred to the second polymerization reactor.
The gist of the seventh aspect of the present invention in the above-described fourth embodiment of the present invention resides in the process for continuously producing a propylene-based block copolymer, wherein the ratio of the polymerization in the first polymerization reactor and that of the second polymerization reactor (first polymerization reactor/second polymerization reactor) in the first polymerization step is 35/65 to 99/1.
The gist of the eight aspect of the present invention resides in a process for continuously producing a propylene-based block copolymer characterized by comprising a first polymerization step wherein an xcex1-olefin comprising liquid propylene as a main component is polymerized in the presence of hydrogen and a stereoregular polymerization catalyst in two polymerization reactors, and a second polymerization step wherein the polymer obtained by the first polymerization step is fed to a copolymerization reactor where propylene and an xcex1-olefin other than propylene are copolymerized substantially in a gaseous phase under the stereoregular polymerization catalyst used in the first polymerization step; and satisfying the following conditions:
(1) The polymer slurry discharged from the first polymerization reactor in the first polymerization step is transferred to the second polymerization reactor;
(2) The polymer slurry discharged from the second polymerization reactor in the first polymerization step is classified by means of a classification system including a classifier into a slurry containing a large amount of particles having a large particle diameter and a slurry containing a large amount of particles having a small particle diameter, and the slurry containing a large amount of particles having a large particle diameter is fed to the second polymerization step, while most part of the slurry containing a large amount of particles having a small particle diameter is recycled to the second polymerization reactor;
(3) A part of the polymer slurry, discharged from the second polymerization reactor in the first polymerization step and classified, containing a large amount of particles having a small particle diameter is fed to a low pressure gas post treatment system; and
(4) The copolymerization of propylene and the xcex1-olefin in the second polymerization step is effected in the presence of a deactivating compound.
The gist of the tenth aspect of the present invention in the above-described eighth embodiment of the present invention resides in the process for continuously producing a propylene-based block copolymer, wherein a part of the slurry discharged from the second polymerization reactor in the first polymerization step and classified, containing a large amount of particles having a small diameter is transferred to the second polymerization step.
The gist of the eleventh aspect of the present invention in the above-described eighth embodiment of the present invention resides in the process for continuously producing a propylene-based block copolymer, wherein the ratio of the polymerization in the first polymerization reactor and that of the second polymerization reactor (first polymerization reactor/second polymerization reactor) in the first polymerization step is 15/85 to 85/15.